Archive for the ‘engineering’ category

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Feeding the WA5VJB cheap Yagis

July 30th, 2010

The WA5VJB cheap Yagis are a great way to get on VHF/UHF without spending a fortune on commercial antennas.  While it is practical on the UHF/microwave bands to use a copper driven element, it is less practical on the 144 and 222 MHz bands.  I know that McMaster carries copper and brass rods, too.  But, I also like to have a coax connector at the feedpoint.  Since I am in the process of building what amounts to a “super cheap Yagi” (note that’s not a “super-cheap Yagi,” the hyphen matters; will report on this in the future), I figured I would share my feedpoint for aluminum driven elements.

While wandering through the electrical aisle of the local big box hardware retailer about six months ago, I discovered the Thomas and Betts ADR6-B2 (try the ADR6 for a drawing of a similar part) grounding lug.  This looked like a good candidate for the cheap Yagi feedpoint, especially costing only $1 for a pair.  In order to fit an SO-239 flange-mount connector to the ADR6-B2, I cut off the portion of the lug with the bolt hole and drilled my own hole (#43) and tapped it 4-40.  I did the same to the other piece.  I did not cut off the lug on the second one, but I should.  Then, I soldered a short piece of wire to the center conductor of the SO-239 jack and added a lug to it.  Here are the parts so far:

Then, I assembled the whole mess on the J-shaped driven element (dummy used for photos) using two 3/8″ 4-40 screws and a lock washer.  Note that the ADR6 lugs are installed on opposite sides of the element.

And, after installing on the wooden boom, it looks like this:

It’s not quite square and some mechanical strengthening is in order before it goes up in the air.  But, this is a considerable improvement over what I’m using now.  Ty-wraping the coax to the boom will provide considerable relief to the connector and it’s attachment.  More details will be forthcoming on the antenna, if it works.  Stay tuned!

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More on the TS-930S PA

July 29th, 2010

I put the “troublesome” TS-930S back on the bench again yesterday to investigate its PA instability some more.  A few points are worth mentioning:

  1. The instability onset power gets lower with increasing frequency.  That is, at 7 and 14 MHz, the instability onset is at about 60 watts output.  At 28 MHz, that drops to 40 watts.
  2. I was able to increase the onset power slightly by increasing the resting bias of the MRF-422/2SC2510A pair from 1.1 A to 1.5 A.  I don’t know if that was a real effect or just a coincidence, though.
  3. I tried adding 0.001 uF, 0.1 uF, and 4.7 uF capacitors shunting the 28B line to ground to remove RF that I observed on that line during transmit.  I don’t have a good measurement of the amount of RF, yet.  But, I know that it’s the same frequency as the transmitter because it varies as I change bands.

I still need to find that Helge Granberg article from RF Design to locate the references.  Still counting my lucky stars that I haven’t destroyed anything in the course of these experiments.

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Noting Differences: Tektronix 464s

July 21st, 2010

Through the usual twists of fate and my inability to pass up a good deal on high-quality used/vintage electronics, I ended up with a pair of Tektronix 464 100-MHz oscilloscopes that were not quite fully functional.  To be correct, one actually belongs to my father, but until it’s working it’s essentially mine.  I did the first check out tonight and noticed something curious when I looked at the nameplate on the backs of the scopes:

Aside from the fact that the first one has Option 4 (increased EMI resistance—in other words, increased PITB factor when assembling/disassembling), what else is different/interesting?  After I noticed it, I was not surprised that one of the units had Option 4…

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Exceeding Tolerance

July 20th, 2010

I read IEEE Spectrum via my Google Reader (and, as an IEEE member, in hard copy).  One of the disappointing things about Spectrum is that it’s written predominantly by people who are science and technology writers, not actual scientists and engineers.  Therefore, it’s a little more sensational than what I’d like (I can read that stuff in Wired).  But, I digress.  I was pleased, however, to see an article today entitled Low Tech Fixes for High Tech Gizmos.

The author wistfully lists all of the things she’s fixed with duct tape and hot-melt glue…

This is really anecdotal evidence of the fact that many non-catestrophic device failures are due to failures of tolerance.  That is, the device (or some of its components) are operating outside their expectations in the design.  This is especially true of mechanical tolerances in low end injection molded consumer devices.  As manufacturers seek to save more, tolerances get tighter.  Cleaning, replacing batteries, disassembly/reassembly, etc, are all examples of the same class of repair.

I just repaired a toy boat for Sarah’s cousin’s son tonight.  It had dead batteries and a gear was pressed too far onto the motor shaft causing the motor to stall.  It’s a good reminder to look for the tolerance failures, even if that sounds like obvious advice.

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IC-290H dial frequency offset

July 5th, 2010

Some time back, I had the good fortune to stumble across a broken IC-290H at an attractive price.  The IC-290H is a synthesized mid-1980s 25-watt 2-meter all-mode transceiver.  Since I had been contemplating a radio to use as the IF to my W1GHZ transverters for 903 through 3456 (yeah, still need to build/integrate all of these), I jumped.  The problem was described as an offset of some tens of kHz between the dial frequency and the actual transmit and receive frequencies.  Since the IC-290 lacks a user community like the TS-600 and TS-700 (my other 2-meter all-mode radio), I went to Google and then posted a quick inquiry on the Stanford VHF e-mail list to see if this was a common problem.  Google produced nothing and the VHF list produced the usual “get the Service Manual” response.  Since I already had the Service Manual thanks to the previous owner, I was set.

The synthesizer (“PLL”) in the IC-290H has at its heart a VCXO (shown above).  The control voltage biases varactor diodes D2 and D3 in a tank circuit with crystal X1 operated between the series and parallel resonant frequencies (as a very high-Q inductor).  Contributions to the control voltage come from the microprocessor (red dot), the receive incremental tuning (RIT, blue dot), and an overall bias of -9 V derived from 5 V using DC-DC converter IC2.

In the “PLL Adjustments” section of the Service Manual, the VCXO adjustments are outlined, checking the synthesizer output with a frequency counter.  I noted that as I changed rotated the VFO encoder on the front of the radio and the digits changed on the display, the output frequency of the synthesizer changed accordingly.  Similarly, the RIT caused the frequency to shift.  Neither of these things were surprising since I could tune in different stations before.  In USB mode at 145.998.5 MHz, the synthesizer should output at 134.250.0 MHz.  I read it at 134.230.21 MHz, clearly the source of the almost 20-kHz offset.  I nudged potentiometer R2 (against the vehement warnings of the Service Manual) and the output didn’t change.  For good measure, I swept R2 over its entire extent with no change.

R2, it seems, controls amount of the available -9 V bias applied to the varactors.  I checked the -9 volts line.  Zero.  Who stole the bias from the cookie jar?  The 5 V line was sagging down to 4.23 V.

I removed IC2, which is in a metal can that resembles an overgrown Mini-Circuits mixer and has only “DP-1” stamped on the cover in black ink.  For kicks, I drove this little guy with 5 volts into no load and got well over -100 V out.  Did this punk destroy other parts in my synthesizer?  I put this question to Dad, who happened to be here over the weekend.  He suggested that it might require a load to produce a regulated output.  Good thinking.  Nothing on the synthesizer board appeared to be charred.  So, my theory was unlikely and I proceeded.

Without IC2, the 5 V line bounced up to 4.95 V.  And, the output side (connected to L6) showed a DC short to ground.  If you trace the circuit from the output of IC2 to one of the legs of R2, nothing should show a DC short.  I looked for bridged solder traces.  None.  Taking the divide an conquer approach, I removed R10 (green dot), to isolate the entire RF portion of the circuit.  Still shorted.  So, this left the following three suspects:  two 0.1 uF ceramic disk capacitors and a 10 V, 100 uF electrolytic.  Since electrolytic capacitors, especially older ones, have a bad reputation for causing problems, I interrogated it first (C20, purple dot).  Bingo.

The only 100 uF capactor I had on hand that even came close to fitting was a 50 V unit from a previous repair.  I squeezed it in—it’s the big brown one against the edge of the case in the center of the photo below.  Note IC2 “DP-1” in the lower right corner.

So, I plugged the radio into a power supply and antenna.  And, low and behold…it was about back on frequency.  So, I completed the synthesizer and RIT adjustments in the Service Manual and put it back together.  It actually receives WA1ZMS/B on 145.285.0, which, Doppler notwithstanding, is GPS-locked at 145.285.000…   Stay tuned for the low-drive transverter IF modification in the next few months once I start building them again.

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50-MHz Progress

June 14th, 2010

As I mentioned in the previous post, I’ve made some strides toward getting on 6 meters over the weekend.  I assembled the 3-element Yagi last weekend.  It was leaning against a post in our back yard pointing skyward for a few days.  I told Sarah that I was thinking of adding 18,431 more of these antennas so I could compete with the 50-MHz radar at Jicamarca.  She was not amused.  I hadn’t even mentioned anything about megawatts.  Yes, Virginia, the mast in the photo is not square.  The top portion of the mast is a little skewed in the rotor and it’s attached with rusty U-bolts that I’ve never loosened.  I’m still turning the antenna with the TR-2, even though I have a T2X out here now.  I will do that swap eventually.  But, for right now, the TR-2 is doing fine.

In order to start moving the transverter toward its new home in a beautiful rack-mount enclosure that previously housed a 900-MHz digital repeater (not included when I obtained the box, unfortunately), I had to “re-arrange its internal organs”, as a menacing extraterrestrial used to say in Space Quest.  Sarah wonders why I schlep all of this crap from place to place with us.  I’ve had that rack-mount box longer than we’ve been married!  It was just waiting for a chance to serve in my shack.  Anyhow.  You can see the layout above.  Sarah says it looks like a doll house.  Pretty sweet house, if I say so myself.

Here’s another view of the partially-integrated box.  I’m running it off a battery because I don’t have the power supply subsystem installed in the box, yet.  It’s really just a PA and some control circuitry away from operational.  Although, I would like to align the TX side with a spectrum analyzer at some point.

I heard quite a few signals in the ARRL contest over the weekend.  W5ZN comes to mind, as well as a couple of locals like N4QQ, who lives just a stone’s throw away on the other side of the Beltway (aka the wrong side of the tracks).  I suspect that the locals would move the S-meter a little more (like past S9) if I put an IF amplifier in after the RX mixer.  But, I’m not really keen to do that unless I have to.

Schematics will come once I’m finished.  But, nothing about this so far has been rocket science (or brain surgery, as the rocket scientists say).  I’ve just been following the Handbook.

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Odds and Ends

May 25th, 2010

Yesterday, I revisited this post listing on-going projects from December 2009.  Some things have changed, some remain the same.

The computer stuff has all been crossed-off the list, except that the home server is off-line with a dead power supply (or motherboard).  I’m somewhat loathe to spend any money on it, but I should be able to pick something up.

While it would probably have been cheaper to buy one of the HF/VHF/UHF combo radios, I’ve set off stupidly down the trail of building (and interfacing) transverters.  I am just three amplifier stages away from having 3-5 watts on 50 MHz!  …plus the interfacing.  I’ve decided that interfacing transverters to radios is more difficult than actually designing and building the transverters themselves.  I built the 903-MHz W1GHZ transverter during the Winter, but haven’t tried it on the air just yet.  W8ISS announced recently that he had some leftovers from the group buy of W1GHZ transverter parts, including boards for 2304 and 3456 as well as some G6Y relay kits.  I bought the lot.  I have enough MMICs and chip caps in the shop to build these and since I’ll need to order a couple of mixers for the other transverters, I can hit the Mini-Circuits minimum order.  Sometime.  Microwaves may all get pushed off to Fall and Winter.

Through a strange coincidence, my wife and I independently decided that it would be a good idea to move my ham shack.  The new location is closer to the center of activity in the house, which means I’ll operate more radio and be more accessible to her while I’m doing it.  But, the feedline and rotator cable no longer reach my 144-MHz Yagi.  Fortunately, I’ll be able to raid the K8GU coax stash shortly.  In the mean time, I’ve been missing what appear from the Hepburn maps to be epic tropo conditions.  Stuff happens.

QSLing, notably my favorite QSL topic—bureau cards.  All bureau requests for KP4/K8GU have been processed.  I ran out of CE/K8GU cards with 10 to go.  I will run some more of these from a photo printer in the next couple of days.  Piles of PJ2/K8GU, K8GU, and (go figure) AA8UP cards remain.  I will get the PJ2 cards done this weekend since I have a box of cards on-hand.  K8GU and AA8UP cards are awaiting a redesign.

Although it sounds like a lot, relatively little is getting done on any of these things thanks to an outdoor project at home.  More on this in the future.

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TS-930S PA mod — first try

May 25th, 2010

As I wrote recently, I have been tackling the low-frequency instability problem in the TS-930S PA unit.  My first attempt was to add several bypass capacitors to from the supply side of L7 to ground (through a lug on the Q7 mounting screw a short distance away).  This should tame the drivers, although the problem is not there.

The hum came back when I pushed the power beyond about 50 watts.  The push-pull MRF-422 final amplifier circuit in the TS-930 comes from Motorola EB27, with a few small modifications.  However, it is not clear to my untrained eye where additional precautions could be taken.  I have a hunch that my next target will be this portion followed by the 2SC2075 pre-driver stage if I come up empty on the finals.

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More on the TS-930S PA

May 21st, 2010

I’m not sure that I’ve shared this on the blog, but I’ve long known that the reason that my “troublesome” 930 burns up PA drivers is a low-frequency oscillation.  This mechanism was also mentioned by Leeson in his comprehensive list of 930 upgrades and repairs.  At first, I thought that the 28-volt power supply’s filter capacitors might be the culprit—insufficient ripple-suppression.  After all, the oscillation sounds like it has a 60-Hz component (I’ve never tried to measure this because I’m concerned about blowing up more transistors).  I finally completed the capacitor replacement last night, realigned the power supply voltage and PA bias levels…and the oscillation is still there, just as I increase the drive past the point where I get 50 watts out.

There is a good Helge Granberg article in the September/October 1980 issue of RF Design titled “Good RF Construction Practices and Techniques.”  I would like to get a copy of the original article because Motorola, in their infinite wisdom, reprinted the article as AR164 but neglected to include the list of references.  Granberg devotes an entire section of the article to instabilities, a full of half of that section to low-frequency instabilities.  This is the area I will focus on first:

Causes for the low frequency instability are usually inadequate collector DC feed bypassing or an extremely poor ground in that area.  Two or three RF chokes together with various values of bypass capacitors from 1000 pF to several uF may be required in the DC line to stabilize the circuit.  (See examples in Reference 1.)

Ah, Reference 1, where (who) art thou?

If I ever get to the bottom of this problem, there should be a useful body of work and notes to prevent this from happening repeatedly to others.

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Transverter Common-IF Box

May 1st, 2010

I weighed the pros and cons of tapping the low-level TX and RX lines inside of my FT-840 (the TS-930 brings these to the rear panel) versus installing an attenuator in the TX line.  I opted for the attenuator instead of doing surgery on the radio.

The circuit is not particularly remarkable:  the transceiver RF jack goes to a relay T/R switch.  The TX side of the relay goes through a hefty 20-dB attenuator to a BNC jack.  The attenuator is rated at 5 watts continuous duty and considerably more at lower duty cycles, perfect for this application.  The RX side goes straight through to a second BNC jack.  The TX circuit is normally closed (and RX normally open).  This minimizes the chance of accidentally dumping 5 watts into the RX side of the transverter.

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